But Downing Street is so serious about the vote on the private member's bill, tabled by the Tory backbencher James Wharton, that it brought forward the prime minister's overseas trip in part to allow him to vote. Tory MPs are facing a three-line whip to support the bill.

Asked whether he would be voting on Friday, the prime minister said: "I will be there. I think this is a serious effort by the government … sorry, the Conservative part of the coalition, to get behind this bill. I totally support it. It is my policy written into law. I would say to all MPs, turn up and vote."